Factor out secure GIC initialisation

Currently the bootwrapper still lumps together logically distinct pieces
of hardware initialisation, making porting to new platforms or adding
new features difficult. It would be nicer if we could separate some of
the functional units to make the code clearer and easier to extend.

To this end, this patch factors the secure GIC initialisation into its
own file. Additionally, the code is modified to route all interrupts to
the non-secure side, not just the first 64.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>